This invention relates to a process for the preparation of N-[4-[[(2-amino-1,4,5,6,7,8,-hexahydro-4-oxo-(6S)-pteridinyl)methyl]amino] benzoyl]-L-glutamic acid (hereinafter called (6S)-tetrahydrofolic acid) and its salts and N-[4-[[(2-amino-1,4,5,6,7,8-hexahydro-4-oxo-(6R)-pteridinyl)methyl]amino]b enzoyl]-L-glutamic acid(hereinafter called (6R)-tetrahydrofolic acid) and its salts.
Tetrahydrofolic acid derivatives contain 2 asymmetric centers. In this case, owing to the synthesis of these derivatives from folic acid, N-(pteroyl)-L-glutamic acid, the optically active C atom contained in the glutamic acid residue is present in the L-form, whereas the optically active C atom in position 6 formed by hydrogenation of the double bond in the 5,6-position of the pteroyl radical is in the racemic, (6R,S)-form. Synthetic derivatives of tetrahydrofolic acid therefore consist of a 1:1 mixture of 2 diastereomers. On natural occurrence, for example in the liver, the tetrahydrofolates are found only in one diastereomeric form, 5,6,7,8-tetrahydrofolic acid being in the (6S)-form.
As medicaments, tetrahydrofolates are mainly used as the calcium salt of 5-formyl-5,6,7,8-tetrahydrofolic acid (leucovorin) or 5-methyl-5,6,7,8-tetrahydrofolic acid for the treatment of megaloblastic folic acid anemia, as an antidote for increasing the tolerability of folic acid antagonists, especially of aminopterin and methotrexate in cancer therapy ("leucovorin rescue"), for increasing the therapeutic effect of 5-fluorouracil and for the treatment of autoimmune diseases such as psoriasis and rheumatoid arthritis and for increasing the tolerability of certain antiparasitics, for example trimethoprim-sulfamethoxazole, in chemotherapy. Tetrahydrofolic acid is used as the basic substance for the preparation of diverse tetrahydrofolic acid derivatives.
Efforts to prepare (6S)- or (6R)-tetrahydrofolic acid have been based on:
enzymatic methods PA1 physicochemical methods PA1 chemical methods
Enzymatic methods comprise reduction, normally carried out chemically, of folic acid to 7,8-dihydrofolic acid and subsequent enzymatic reduction thereof to (6S)-5,6,7,8-tetrahydrofolic acid, for example according to L. Rees et. al., Tetrahedron 42(1), 117-36 (1986) or EP-A2-0,356,934. However, these processes can only be stopped with difficulty in the chemical step at the 7,8-dihydrofolic acid stage and also typically give only very small space-time yields in the enzymatic step, require expensive co-factors such as NADPH and necessitate an usually complex working-up methodology. Methods for the enzymatic preparation of optically pure tetrahydrofolic acid known hitherto are not suitable for the preparation of this compound on the industrial scale.
The separation of the diastereomer pairs was also attempted by means of chromatography, J. Feeney et. al., Biochemistry, 20, 1837, (1981). These methods are not suitable for the preparation of the diastereomers on the industrial scale.
An asymmetric reduction of folic acid on chiral electrodes is also known from the group of physicochemical processes, S. Kwee et. al., Bioelectrochem. Bioenerg. 7, 693-698, (1980). Owing to the concentrations of folic acid (typically 10.sup.-3 M) permitted during the reduction and the removal of the asymmetric inductor, which can only be carried out with difficulty, after reduction has taken place, these reactions, however, cannot be employed for industrial preparation.
From the field of chemical synthesis, the possibility of asymmetric hydrogenation of folic acid in the presence of an optically active catalyst exists, for example according to P. H. Boyle, et. al., J. Chem. Soc. Chem. Commun. (1974), 10, 375-6. However, this requires the use of very expensive catalysts, which, after homogeneous catalysis has taken place, can only be separated off with great loss of the product.
There is to date therefore still no industrially utilizable process for obtaining optically pure tetrahydrofolic acid.